Actually, EVERY created being is a "son of God" since He created all who live, whether they became evil or not.
We just don't know now WHO the 'sons of God' were who bred with human women and produced giants. We just know it did happen, and that they were apparently superior to humans, as they produced giants far-larger and more-powerful than ordinary men.
And we don't know JUST WHEN these 'sons of God' came to earth, even though the wording of Gen. 6:1-4 suggests it mighta been after several generations of people had been born. But again, I don't believe God EVER condoned incest between full biological siblings.
And we know they returned after the flood, as once again there were giants in the land. And these giants were far-larger than any 'giants' of today. For example, there are many Israelilegends saying King Og of the Amorites was twice the size of a normal man, and a stone burial mound in the ruins of ancient Rabbah, near Amman, Jordan, has a resting place in it of some 13 feet in length, as compared with about six feet for the others. (Not to mention the giants of Saul's and David's time.)
Now, given the long lifespans of the early homo sapiens, it's possible that Cain's wife was the product of one of his sisters and a 'son of the gods', which would fall within the parameters of what GOD once condoned incest-wise. But I don't believe He EVER condoned sex between full-blood siblings, or between parent and child. While He didn't immediately destroy Lot's daughters for their incestuous acts, I don't believe He held them guiltless. Met any Moabites or Ammonites lately?